Carbon fiber has been used in the application of aircraft since it has high specific strength and specific modulus. More specifically, carbon fiber has been used as a fiber for reinforcing fiber-reinforced composite materials and contributed to weight reduction of aircraft. A recent growing trend is wider use of the member including the carbon fiber and use of the carbon fiber in a larger member. The most effective means for weight reduction of aircraft may be improvement of the carbon fiber in the aspect of tensile modulus which controls the rigidity of the carbon fiber-reinforced composite material. However, there is also a requirement for a good balance between a wide variety of mechanical properties including tensile and compressive strength and open hole tensile and compressive strength as a carbon fiber-reinforced composite material. In particular, when the carbon fiber-reinforced composite material is used for aircraft applications, the open hole tensile strength is more important than the tensile strength of the unidirectional carbon fiber-reinforced composite material since use of a drilled pseudo-isotropic material with a fastener is prevalent.
There are many factors that influence the open hole tensile strength, and the mechanism of strength development involves many unclear aspects. However, the general conception of the effects of the carbon fiber on the open hole tensile strength has been such that the open hole tensile strength is proportional to the tensile strength of resin-impregnated strands of the carbon fiber. The “tensile strength of resin-impregnated strands” is an index adopted in view of the convenience of examining the strength potential of the carbon fiber which is the reinforcing fiber. More specifically, the “tensile strength of resin-impregnated strands” is the tensile strength of the simple unidirectional carbon fiber-reinforced composite material prepared by impregnating a particular epoxy resin (hereinafter referred to as strength of the unidirectional composite material).
Some investigations have examined the properties of carbon fibers for the purpose of improving open hole tensile strength of the carbon fiber-reinforced composite material (Japanese Unexamined Patent Publication (Kokai) No. 2010-047865 and Japanese Unexamined Patent Publication (Kokai) No. 2010-111957). Japanese Unexamined Patent Publication (Kokai) No. 2010-047865 discloses an attempt wherein surface morphology of the carbon fiber and the surface treatment conditions of the carbon fiber are changed to improve the open hole tensile strength of the carbon fiber-reinforced composite material. Japanese Unexamined Patent Publication (Kokai) No. 2010-111957 discloses the idea of controlling spreadability of the carbon fiber and its surface wettability to improve the open hole tensile strength of the carbon fiber-reinforced composite material. The open hole tensile strength, however, remained at a low level.
Recently, some techniques of conducting the carbonization at a high draw tension have also been proposed to improve the tensile modulus of the carbon fiber not by controlling the maximum temperature in the carbonization process (Japanese Unexamined Patent Publication (Kokai) No. 2008-248219, Japanese Unexamined Patent Publication (Kokai) No. 2008-308776, and Japanese Unexamined Patent Publication (Kokai) No. 2008-308777). Japanese Unexamined Patent Publication (Kokai) No. 2008-248219 discloses that, when the polyacrylonitrile polymer used in the production of the carbon fiber has a particular molecular weight distribution, the resulting carbon fiber will exhibit high tensile strength and modulus of the resin-impregnated strands in normal range of conditions. In Japanese Unexamined Patent Publication (Kokai) No. 2008-308776 and Japanese Unexamined Patent Publication (Kokai) No. 2008-308777, the focus is on the tensile modulus of the carbon fiber, and the single-fiber strength of the carbon fiber has not been controlled. In addition, since the draw tension is increased in the process of carbonization the pre-carbonized fiber bundle, loss of the quality has been inevitable and the open hole tensile strength has also remained at a low level.
Japanese Unexamined Patent Publication (Kokai) No. 2004-316052 proposes a technique wherein the precursor fiber bundle of the carbon fiber is subjected to a high level drawing in the oxidation process and the pre-carbonization process in an attempt to improve tensile modulus of resin-impregnated strands. In that technique, however, drawing is conducted before carbonization, and influence on the carbon fiber structure was minimal. Moreover, that technique is not the one controlling the single-fiber strength of the carbon fiber.
Japanese Unexamined Patent Publication (Kokai) No. HEI-11-12874 and Japanese Unexamined Patent Publication (Kokai) No. 2009-114578 propose the technique of interlacing the precursor fiber for the purpose of preventing pseudo-adhesion by the oiling agent in the spinning process. However, that technique was far from simultaneously realizing the tensile strength of resin-impregnated strands and the tensile modulus of resin-impregnated strands at a high level.
Also proposed is a technique wherein single-fiber diameter of the carbon fiber is controlled to the small diameter range to reduce the probability of surface flaw generation to improve single-fiber strength of the carbon fiber (Japanese Unexamined Patent Publication (Kokai) No. HEI-11-241230). While that technique can realize the high tensile strength and modulus of the resin-impregnated strands, variation in the structure between the single-fibers and the associated variation in the strength between the single-fibers are induced in the carbonization process. In addition, fluffing and fiber breakage are induced in the carbonization process, and this inevitably resulted in the inferior operability and unfavorable quality of the resulting carbon fiber bundle.
We found that, when a carbon fiber having an excellent tensile modulus and a particular matrix resin capable of developing very high open hole tensile strength are combined, open hole tensile strength (hereinafter sometimes abbreviated as OHT) of the resulting carbon fiber-reinforced composite material is not improved even if the tensile strength of the resin-impregnated strands of the carbon fiber is increased and, therefore, an approach entirely different from conventional approaches is needed to achieve a carbon fiber-reinforced composite material having a higher open hole tensile strength. Accordingly, it could be helpful to provide a prepreg containing a carbon fiber having an excellent tensile modulus capable of producing a carbon fiber-reinforced composite material having a high open hole tensile strength, and a sizing agent-coated carbon fiber bundle usable in such prepreg.
It could also be helpful to provide a carbon fiber bundle simultaneously having a high tensile strength and a high tensile modulus of the resin-impregnated strands which also has excellent quality.